Hydraulic tensioners which include a check valve mechanism been widely used to maintain proper tension in timing chains and timing belts which transmit rotation from a crankshaft to one or more camshafts in a vehicle engine. The check valve mechanism is used to prevent vibration generated during the operation of the chain or belt.
A conventional hydraulic tensioner typically includes a check valve mechanism comprises a metal cylinder fitted into a cylindrical plunger-receiving hole formed in a housing, a plunger slidably received in the hole and having an end portion protruding from the housing and biased in the protruding direction by a spring inside the housing, and a high pressure oil chamber formed between the inside of the plunger and the plunger-receiving hole in the housing. The check valve mechanism may be assembled prior to installation in the tensioner housing, and comprises cylindrical block, a ball seat having an oil passage, a check ball, a coil spring and a lid. Such a check valve mechanism is shown in Japanese Laid-Open Patent Publication No. 2002-206603 (Page 1, FIG. 1).
The above-described check valve mechanism requires high accuracy machining, and consequently its production cost was high, and difficulties were encountered in the assembly of the tensioner.
A related hydraulic tensioner, invented by the inventor of the present invention, is shown in FIGS. 6 and 7. In the tensioner 500, a plunger 520 fits slidably in a plunger-receiving hole of a housing 510, and is biased in the protruding direction by a coil spring 530 situated in a high pressure oil chamber R formed by the plunger and the housing. The tensioner includes a check valve unit 540, which controls the flow of oil into and out of the high pressure oil chamber. The check valve unit comprises a check ball 541, a ball guide 542 in which the check ball 541 moves, a disc-shaped retainer 543, which seals the check ball 541 in the ball guide 542, and a ball seat 544, which blocks reverse flow of oil by seating the check ball 541.
When the hydraulic tensioner 500 is arranged to apply tension to a chain on an engine in such a manner that its plunger 520 protrudes downward to abut the chain, air accumulated in the high pressure chamber R is not adequately ventilated. Consequently, the force exerted by the plunger on the chain is sometimes insufficient. In such a case, backlash noises are generated by the chain.
Objects of this invention are to solve the above-mentioned problems, and to provide a hydraulic tensioner in which, even when the plunger protrudes downward, adequate air ventilation from the high pressure chamber of the tensioner is provided so that backlash noises are suppressed and leak down of oil from the high pressure oil chamber can be controlled. It is also an object of the invention to provide a hydraulic tensioner that has the above advantages, but can be produced easily.